The present invention relates to iron base alloys with improved wear resistance and hot hardness, and also relates to iron base alloys containing primary intermetallic compounds as hard phases. Such alloys are particularly useful for internal combustion engine components, such as valve seat inserts, etc. In a further aspect, this invention relates to components made from such alloys, either cast or hardfaced. An example is a hardfacing for a valve for an internal combustion engine. Alternatively, components made of such alloys may be made by conventional powder metallurgy methods, either by cold pressing and sintering, or by hot pressing at elevated pressures for wear resistant applications.
The development of dry fuel internal combustion engines and tighter emission standards for diesel fuel internal combustion engines require better wear resistant alloys for valve seat insert applications. M2 tool steel (by AISI designation) and high carbon, high chromium type steels are commercially available alloys for valve seat inserts. However, often these alloys experience severe wear in such applications. Stellite(copyright) and cobalt base Tribaloy(copyright) are alternative alloys used in these situations in the industry.1 Unfortunately, these alloys are expensive due to the high cost of the cobalt included in the alloys.
1 (copyright)Registered Trademark of Deloro Stellite Company Inc. 
Increasing the hardness of iron base alloys can effectively improve wear resistance of valve seat inserts in many instances. However, this approach also increases machining difficulties and therefore manufacturing cost of valve seat inserts. It would be desirable to have an alloy that increases its hot hardness and wear resistance at working conditions, to avoid machining difficulties associated with conventional high hardness alloys.
Many patents disclose alloy steel or tool steel-type iron base alloys for wear resistant applications. U.S. Pat. Nos. 4,778,522 and 4,844,024 disclose a few examples of tool steel-type wear resistant alloys, where a certain amount of carbon is required to form alloy carbides for better wear resistance. Therefore, carbon is an essential element and is normally higher than 0.1 wt % in these alloys.
U.S. Pat. Nos. 3,257,178, 3,361,560 and 3,410,732 disclose another type of wear resistant alloys which include intermetallic compounds, such as Laves phase, as hard phases for wear resistant applications. Many alloy systems, such as Wxe2x80x94Nixe2x80x94Si, Moxe2x80x94Nixe2x80x94Si, Wxe2x80x94Coxe2x80x94Si and Moxe2x80x94Crxe2x80x94Nixe2x80x94Si, were tested as protective coatings (U.S. Pat. No. 3,257,178). Cobalt base alloys, composed of 14-30 wt % molybdenum, 6-12 wt % chromium, 0.54 wt % silicon, and at least 50 wt % cobalt, are disclosed in U.S. Pat. No. 3,410,732.
U.S. Pat. No. 4,933,008 discloses sintered iron base alloys comprised of a high speed tool steel matrix powder and a hard alloy powder. The two types of powders were mixed and sintered to form wear resistant alloys. The hard alloys are composed of 0.02-0.2 wt % carbon, 3-30 wt % silicon, 0.05-0.7 wt % manganese, 10-60 wt % molybdenum, 1-7 wt % titanium, 0.5-2 wt % boron, 1.0-10 wt % nickel, and the balance being iron and impurities. Japanese Patent Publication No. 53-112206 discloses additional sintered iron base alloys with additions of 540 wt % of hard alloy to increase wear resistance of the powder metallurgy alloy. The hard alloys have the following chemical compositions: less than 0.01 wt % carbon, 0.5-1.0 wt % silicon, less than 0.4 wt % manganese, 10-50 wt % molybdenum, and less than 40 wt % combined total of nickel, chromium and/or cobalt.
These last two references describe the method of making the alloys by adding a certain amount of hard alloy powder to a matrix alloy to make wear resistant materials through a powder metallurgy process. The powder metallurgy process makes it possible to use high molybdenum (up to 50 wt %) and high silicon (up to 30 wt %) to form very fine alloy particles without a cracking problem. However, it is very difficult for conventional casting methods to use such compositions as casting alloys to produce engine components, because silicon and molybdenum contents in such high levels make the components extremely brittle.
It would be beneficial if an iron-based composition could be developed for internal combustion engine components because it would have a low cost compared to nickel and cobalt alloys. An intermetallic-type iron base casting alloy for wear resistant application is thus highly desirable. An alloy with precipitation hardening ability at exhaust working temperatures to improve the service life of valve seat inserts would be a further significant improvement.
Iron base alloys have been invented that have good wear resistance. One aspect of the invention includes alloys that have a unique feature of hardening when exposed to working temperatures of exhaust valve seat inserts, which in turn improves wear resistance and hot hardness of the alloys. The hardening of the alloys is achieved through controlling chemical compositions of the alloys to promote precipitation of secondary intermetallic compounds at relatively low temperatures. The alloys also have high sliding wear resistance and high hardness at elevated temperatures, and the cost of the alloys is significantly lower than commercially available cobalt base alloys, such as Stellite(copyright) and Tribaloy(copyright). In one aspect, the present invention is an alloy with the following composition:
Optionally, the inventive alloys do not need to contain cobalt when they are to be used for applications where the working temperature is too low to generate any hardening effect in the alloy, such as 600xc2x0 F. (316xc2x0 C.). For this use, the inventive alloys preferably have the following composition:
Further optionally, the inventive alloys contain nickel in an amount of 3-14 wt % to totally or partially replace cobalt for applications where the working temperature is too high (over 1000xc2x0 F.) (538xc2x0 C.) to keep the hardening effect promoted by cobalt in the alloy. Under such conditions, molybdenum content should be limited to 26-36 wt % in order to achieve maximum wear resistance of the inventive alloys. In this aspect, the preferred inventive alloys have the following composition:
In another aspect of the invention, metal components are either made of the alloys, such as by casting, or using powder metallurgy methods, such as by forming the component from a powder and sintering. Furthermore, the alloys may be used to hardface components as a protective coating.